Homeobox genes are an ancient family of developmental regulators that pattern animal embryos. We are fascinated by the genetic circuitry involving homeobox genes that assigns developmental fates to different cells in embryos, and how the redeployment of such circuits has helped to evolve morphological diversity during animal evolution. One aim is to test the idea that a function of microRNAs is to dampen the deleterious effects of sporadic transcription of developmental regulatory genes, such as those of the homeobox family. Another aim is based on our finding that genes of the core proximodistal appendage- patterning network of arthropods are expressed in the anterior neuroectoderm of Drosophila embryos, as well as in the anterior neuroectoderm of chordate embryos in an overlapping anteroposterior order. These genes encode the transcription factors Distal-less, apterous, dachshund, hemothorax, and buttonhead/Sp8. These results, in concert with existing expression data from a variety of other animals, including mammals, suggest that a pre-existing gene network for anterior head patterning, which eventually results in different regional specializations of the brain, was co-opted to pattern the proximodistal axis of bilateral appendages in animals. This model needs more experimental tests, and such tests are outlined in this proposal. These include testing for conserved cross-regulatory relationships among these genes in the anterior neuroectoderm of Drosophila embryos, and testing whether the same set of genes are expressed in the heads of animals that represent the common ancestors of present day arthropods and vertebrates. If validated, this model would represent an amazing example of how a gene network has been redeployed to innovate new morphological features in animal body plans. Another aim is to use high resolution in situ hybridization to study how certain homeobox genes are transcriptionally activated by long-range DNA enhancers at the level of individual genes in individual embryonic nuclei.